Together, these two graphs provide a record of Americans’ driving habits over a quarter century. And the story they tell is remarkable. Between 1977 and about 2004, VMT and VMT per capita rose steadily at a rate of about 2.7 and 1.5 percent per year respectively. There were some minor reversals in the trend: for four years during the late ‘70s and early ‘80s, a slice of time that saw the oil shock and accompanying recession, VMT noticeably dipped; the impacts of the smaller recessions of 1990 and 2001 can also be discerned. But with those three exceptions, the trend in vehicle miles traveled during the 25-year period preceding 2003 was upward and pretty steeply upward.

And then something happened. Starting in about 2004, the growth in VMT slowed and VMT per capita essentially went flat. (See Exhibit A.) In 2006, VMT per capita began falling, and in 2008, when the great recession hit, VMT also took a hit. Since 2009, VMT has been essentially flat, bouncing up a bit one year and then down the next, while VMT per capita has continued to fall. (See related photos: “Cars That Fired Our Love-Hate Relationship With Fuel.”)

Clearly, the pattern we’ve seen since around 2004 is fundamentally different from the pattern of the 20+ previous years. The question is why.

But I don’t think we’re seeing a driver response to hard economic times. For one, the slowdown in the rise in VMT and the fall in VMT per capita began well before the great recession. Second, take a look at this New York Times graphicNew York Times graphic plotting VMT and gross domestic product (GDP) over a 50-year span through 2010. Over the first 35 years, VMT and GDP tracked each other year after year. Then around 1997, the GDP and VMT lines begin to diverge with GDP increasing more rapidly then VMT. To me this is rather convincing evidence that something more fundamental is going on — an unprecedented decoupling of American driving habits and macroeconomic forces.

(By the way, another thing that has been going on of late is that fuel economy has been increasing and electric vehicle sales have been climbing — see yesterday’s post — and all these things working together are really having an impact on emissions.)

Could It Be Pain at the Pump?

The economy writ large may not explain the change in VMT, but a specific economic factor could. A likely candidate: the price of gasoline. In support of that theory, I present Exhibit C — a plot of VMT and gasoline prices since 1970. Two things to note:

In inflation-adjusted dollars, the price of gasoline since 2008 (~$2.90/gallon in 2005 $s) is higher than it has ever been; and

Sources: Travel data are from the Federal Highway Administration. Gas price data, which are in real 2005$ adjusted for inflation, are from the U.S. Energy Information Administration. 1970-1975 prices are for leaded regular gas, 1991-2011 prices are for unleaded regular gas, and 1976-1990 prices are the average of both leaded and unleaded gasoline.

One interpretation: we have hit a price threshold; a price so high that people are forced to find alternative ways to get where they need to go. If this is the case, here again, what we are seeing may not be a permanent change in American driving habits; should gasoline prices fall, VMT will probably start moving upward.

But there is another possibility.

From Automobiles to Trains and Buses

In the early 2000s quarterly mass transit ridership was between 2.3 and 2.4 million. But late in 2003, coincident with the beginnings of the change in VMT trend (and the rise in gas prices), ridership began trending upwards before settling into a new steady state of between of 2.5 to 2.7 million riders since 2008. Could this switch to mass transit be symptomatic of a more fundamental change in America?

High gasoline prices might be forcing some of these changes on us to some extent, but the new trend in VMT illuminates a picture in which the change in driving habits is becoming internalized and may be becoming long-term. (See quiz: “What You Don’t Know About Gas Prices.”)

And on that note, here’s an interesting thought: with both U.S. greenhouse gas emissions and VMT running flat since 2008, it could be that we are seeing a profound change in the United States. A change in the American way of life that is less bent on both consumption and driving. And a way of life ready to embrace a low-carbon economy. Now there’s a nice idea for the New Year. (See related photos: “Rare Look Inside Carmakers’ Drive for 55 MPG.”)

________________End Note

* Keep in mind that while recessions and miles traveled correlate well, this doesn’t tell us what initiated these recessions. Both recessions from the ’70s began with oil shocks where the supply of petroleum was curtailed and prices rose sharply. And even the smaller recessions in 1990 and 2001 were preceded by an increase in energy prices.